Pellicles are membranes used during lithography. A pellicle is placed a desired distance from either the front side or the backside of a mask. Pellicles may be used to block particles that are in the focal plane from reaching the mask or reticle surface. Any particles on the pellicle surface are out of the focal plane and hence should not form an image on the wafer being exposed. A pellicle is a thin transparent layer stretched over a frame above the surface of a mask or reticle. Typically the pellicle is transparent to laser light. Applied laser energy will depend on pellicle and resist transmission. For example, critical dimensions of the printed resist features are very sensitive to the variation dose of laser energy. A 2% difference in dose can result in 10% variation in critical dimensions.